Heat, illness, older relatives, pregnancy, hearts and kidneys, and when it's more than a dry day

The rule is for an ordinary adult on an ordinary day. Here are the days and the people it isn't for, and what the NHS says instead.

Heat, exercise and illness. The NHS says you need more if you're in a hot environment, physically active for long periods, ill or recovering, and that water is the best way to replace fluid lost through sweat. If you've been sick or had diarrhoea, you may need to put back salts and sugars as well, and a pharmacist can recommend an oral rehydration solution. On those days the pair isn't enough and you should drink beyond it, and avoid caffeine and alcohol while you're symptomatic, as the dehydration page advises.

The urgent list. The NHS says to ask for an urgent GP appointment or call 111 if you're feeling unusually tired, if you feel dizzy when you stand up and it doesn't go away, if you have dark yellow pee or are peeing less than normal, or if you're breathing quickly or have a fast heart rate. And call 999 or go to A&E if there are signs of dehydration together with cold skin, blue, grey, pale or blotchy skin or lips, difficulty breathing, confusion or unusual sleepiness. That's dehydration and not a dry day, and a glass of water is not the response.

Older relatives. Thirst gets less reliable with age. Phillips, Rolls and colleagues deprived seven healthy men aged 67 to 75 and seven aged 20 to 31 of water for 24 hours. The older men were less thirsty and drank less afterwards, even though their blood showed a bigger deficit. The NHS lists older adults among the people most at risk and gives the practical advice. Make sure they drink at mealtimes, make drinking a social thing, offer food with water in it, soup and melon and jelly, and keep fluid within reach at all times. If you're the older adult, the pair matters more for you than for anyone else in this book, because your quiet signal has got quieter.

Pregnancy and breastfeeding. More fluid is needed, the NHS says, and caffeine should be kept under 200 milligrams a day in pregnancy. The pair works, with the hot drinks counted.

Hearts and kidneys. Some people with heart failure or kidney disease are told by their doctors to restrict fluid. If that's you, this book doesn't override that, and the pair is not for you unless your doctor says so.

Diabetes and diuretics. Both make dehydration more likely, the NHS says. The rule still applies, and the pale check matters more, and dark pee plus thirst plus tiredness is a GP conversation and not just a dry day.

When the afternoon isn't the water. If you've done the pair and the glass first for a month, your pee is pale, and the headache and the flatness are still there most days, then the water wasn't the cause, and it's worth ruling out the other ordinary ones, sleep, caffeine timing, eyesight, and if those don't answer it, a GP. This book is one cheap thing to try first, not a diagnosis.